The present invention concerns a method for measuring the distribution in the thickness direction of filler and/or coating materials of paper, cardboard or the equivalent, and the contents of said materials without destroying the sample. In the method or procedure of the invention, the radiation emitted by an x-ray source is used to excite in the material component under examination of the object under measurement its characteristic x-ray radiation and the intensity of this radiation is observed. In procedure, measurements are carried out on both sides of the specimen under examination. In addition, the contents of other filler components are determined by x-ray radiation absorption measurements in order to eliminate the effects of such components interfering with the distribution measurement, and the base weight of the paper is determined by beta radiation absorption measurement, or by another equivalent procedure.
Furthermore, the present invention concerns apparatus for applying the method and novel uses of the procedure and the apparatus.
When paper and paper machines are discussed in the following, reference is generally made both to paper and cardboard, and respectively both to paper and cardboard machines.
Fillers, which as a rule are mineral substances, are incorporated in the paper primarily for their effect of improving the printing technological properties. Fillers are most commonly used for printing papers. The filler addition improves the opacity, lightness, printer's ink absorption and surface smoothness of the paper.
The fillers influence in a particularly advantageous manner the quality of paper to be glazed.
It is known in the art to add filler material in two ways, either by mass filling or by coating. In the mass filling method or procedure, the filler material is added in the form of suspension to the pulp sludge before the arrival of the sludge on the paper machine, whereby the filler material is admixed with the entire fiber material in the finished paper. In the coating procedure, a suitable glue substance is admixed with the filler material in the aqueous phase, such as starch or casein, whereafter the surface of the paper is brushed with this mixture in a continuous process.
The filler materials in paper tend to be non-uniformly distributed in the thickness direction of the paper, causing one-sidedness of the paper. The one-sidedness of paper manufactured on Fourdrinier machines is due to the fact that the fillers are washed out together with the water that is drained, from the lower part of the pulp web into the drainage water, whereby they become enriched in the upper part of the web. As is known in the art, efforts have been made to reduce the problems of one-sidedness, not only by additives improving the retention, but also by gentle dewatering at the initial draining phase, which requires a longer dewatering time and therefore implies lengthening the wire section or reducing the speed of the paper machine.
In machines with a planar wire, the difficulties with the fines and filler distribution manifest themselves when papers for offset printing are manufactured. A high filler and fines content on the top surface of the paper causes dusting, which is a serious detriment in the offset process. In contrast, papers manufactured on a twin wire machine are considered well appropriate for offset printing. This is due to the symmetrical shape of the fines distribution and to equal leaching of both surfaces of the web due to two-sided dewatering. It is in fact generally held that due to more uniform fines distribution, the printing by offset on paper manufactured on a twin wire machine is more successful than that on paper manufactured on a Fourdrinier machine. Offset printability has indeed increased in significance because offset printing is increasingly replacing the letterpress printing procedure.
On the other hand, the filler content of the surfaces of the paper web cannot always be brought to a desired level with a twin wire former. Even when planar wires are used only the top side of the web (the side facing away from the wire) is satisfactory as to its filler content. The low filler content of the web surface is particularly problematic in so-called SC gravure papers. Attempts may be made to increase the filler content of the paper surfaces by increasing the filler content of the pulp in the headbox, but even with this expedient, a satisfactory condition is not achieved because of the above-mentioned poor retention characteristic of the filler and of its enriching in the inner parts of the paper. In addition, when the filler content in the headbox has to be increased, the consistency in the headbox is likely to become excessive so that it impairs the formation of the paper.
Modern high-speed printing presses impose particularly high requirements on the printing paper. These requirements are based on trouble-free operation of fast printing presses and on the appearance of the printing. The imprint is considerably influenced by the symmetry between the sides of the paper and the quality of the surfaces of the paper, which is naturally also influenced by the distribution of the fillers. Heretofore no methods or procedures and apparatus have been in use with which the filler distribution could have been measured even on line either in the paper machine, in the printing press or in the paper coating means.
It is known in the art, as described in Finnish Pat. No. 40587, inventors Juhani Kuusi and Antti Lehtinen, applicant Valmet Oy, to excite the characteristic x-ray radiation of the filler material by radiations such as alpha, beta, gamma or x-ray radiation, penetrating to various depths in the paper, and in this ways to gain information on the vertical distribution of the filler. The procedure has been described in greater detail in a paper by J. Kuusi, entitled "Determination of Content and Distribution of Filler and Coating Materials in Paper Using Radioisotope X-Ray Spectrometry", Paper and Timber No. 4A 1970. As was observed in the paper, variations in relation to each other of the filler contents cause certain effects of which the quantitative elimination by the procedures described in the paper is impossible. This has impeded the introduction, practice of such procedures.
The state of the art regarding filler measurements is illustrated in general by a publication of April, 1982 by Buchnes A. McNelles L. A. and Hewitt J. S., entitled "The Application of X-Ray Absorption and Fluorescence Analysis to the Measurement of Paper Additives", Int. J. Appl. Radiat. Isot. Vol. 33, pp. 285 to 292 (1982), where a fluorescence and absorption technique is used for determining the total contents of different fillers, based on the assumption that the fillers are uniformly distributed in the thickness direction of the paper. In practice, this is hardly ever the case. It is thus understood that in this application, and the references cited therein and in its author's patent for "On-Line System for Monitoring Sheet Material Additives", U.S. Pat. No. 4,081,676, March, 1978, no endeavors whatsoever were made to determine the important thickness-direction distribution, nor has it even been taken into account as a potential source of error in determination of the total filler content. It should be noted, however, that in the instances described in the paper, the influence of said source of error is minimal.
Procedures or methods capable of determining the filler distribution and the total filler content directly in the paper machine are not in use at all.